This invention relates to a sheet feeding apparatus for forming a buckle in a sheet and to a reproducing machine employing such an apparatus and to the process.
It is known in the sheet feeding art that numerous beneficial effects can be obtained by buckling a sheet during feeding. Sheet buckling has been employed as a device for improving sheet separation from a stack of sheets. A typical example of such a use of sheet buckling would be the reverse buckle sheet feeder wherein a sheet is first fed rearwardly against a stop member to form a buckle to separate it from the next adjacent sheet of the stack and then forwardly into the appropriate sheet feed path. This type of approach is used commercially as, for example, in the sheet feeder of the Xerox 3100 copier.
Yet another purpose of buckling sheets in a sheet feeding environment is to remove residual skew which may have occurred during initial feeding. Here again a sheet is buckled up against a stop member. Then it is gripped while buckled for further feeding to eliminate residual skew. A typical example of the implementation of this type of buckling approach is also found in the Xerox 3100 copier wherein the sheet after feeding from the stack is fed up against a registration gate and forward buckled to remove residual skew.
It has been found that particularly when buckling to achieve the removal of residual skew during sheet feeding the buckle height must be controlled to obtain consistent skew removal. A variety of approaches have been adopted by the prior art for controlling buckle height. In the Xerox 3100 copier reverse buckle feeder fixed feeding times are utilized for controlling the buckle heights. This type of approach has also been utilized in U.S. Pat. No. 1,570,592 for a forward buckling type sheet feeding apparatus. These approaches, while adequate for their intended purposes, leave considerable room for improvement. Utilizing a fixed feeding time for forming a buckle fails to take account of slippage between the feeder and the sheet, and differences in the position of the lead edge of the sheet at the time the feeding cycle commences.
Other approaches have been adpoted in the prior art for controlling buckle height in a sheet feeder which overcome the problems of utilizing a fixed feeding time. In U.S. Pat. Nos. 3,241,831; 3,270,787, and 3,335,662, there are disclosed apparatuses wherein the height of the buckle is controlled by sensing the height electrically as by the use of a photodetector or lever switch. After the appropriate buckle height has been sensed the sheet is then fed in a conventional manner.
The use of an optical sensor or a lever type switch for sensing buckle height is disadvantageous because it is difficult to accurately locate the sensor in the sheet feed path. Further, it has been found that the buckle in different kinds of sheets does not necessarily form at the same point in the sheet feed path. An illustrative example of this problem would be feeding a sheet of labels to an imaging device such as a copier. A sheet of labels usually comprises a flexible backing sheet with a plurality of labels adhesively arranged on one side of the sheet. When buckling such a sheet there is greater flexibility in those portions of the sheet between labels than in those portions of the sheet where a label is present. Therefore, the buckle will often form upstream or downstream of a buckle which would form in the sheet if no labels were present.